This invention relates to composite polymeric materials having metallic surface properties, and, more particularly, to composite polymeric materials formed with an epitaxial crystalline film of polymeric sulfur nitride.
The covalent polymeric metal, polymeric sulfur nitride, (SN).sub.x, also known as polythiazyl, is believed to be the first known example of a metallic polymer. In single-crystal form, polymeric sulfur nitride exhibits a highly anisotropic electrical conductivity with a metal-like temperature dependence, as well as optical properties which are similarly anisotropic and suggestive of metallic behavior.
The synthesis, structure and properties of analytically pure single crystals of polymeric sulfur nitride have been described by Mikulski, et al, J. Amer. Chem. Soc., Vol. 97, 22, Oct. 29, 1975, pages 6358-6363. The method of synthesis described by Mikulski, et al, involves first the production of pure S.sub.2 N.sub.2 from S.sub.4 N.sub.4 by passing S.sub.4 N.sub.4 vapor over heated silver wool to form S.sub.2 N.sub.2 vapor, condensing the S.sub.2 N.sub.2 on the surface of a cold finger containing liquid nitrogen, and then bringing the liquid nitrogen cold traps to room temperature to permit the S.sub.2 N.sub.2 to sublime slowly from them into a rectangular trap held at 0.degree. C. The sublimation step assists in separating the S.sub.2 N.sub.2 from any less volatile impurities and also permits the slow growth of good S.sub.2 N.sub.2 crystals. When appropriate size S.sub.2 N.sub.2 crystals have been obtained, they are permitted to polymerize spontaneously in the solid state at room temperature over a period of about 60 hours, followed by heating at 75.degree. C. for about two hours to complete the polymerization. This procedure results in the production of brillant, highly-lustrous, golden crystals of polymeric sulfur nitride which are pseudomorphs of and have the same space group as the S.sub.2 N.sub.2 crystals from which they are derived. These polymeric sulfur nitride crystals are composed of an ordered array of parallel polymeric sulfur nitride fibers which consist of an almost planar chain of alternating sulfur and nitrogen atoms. The product is analytically pure, and x-ray diffraction shows the absence of S.sub.4 N.sub.4 and S.sub.2 N.sub.2.
The highly anisotropic electrical conductivity and optical properties exhibited by the single-crystal form of polymeric sulfur nitride makes this metallic polymer potentially useful in the form of a film fabricated on a suitable substrate so as to form a composite material having metallic surface properties. Such composite material would have a wide variety of useful applications taking advantage of the combined mechanical, electrical, and optical properties thereof, such as, for example, in integrated optical systems. However, previous attempts along these lines have failed to produce a polymeric sulfur nitride film having the expected high degree of anisotropy with respect to its electrical conductivity and optical properties, apparently due to a random and incomplete alignment of the polymeric sulfur nitride chains in the film. For example, when polymeric sulfur nitride films are fabricated by direct sublimation of the crystalline polymeric sulfur nitride onto a glass substrate, the resulting films, when examined under a microscope, have the same gold appearance as crystalline polymeric sulfur nitride and exhibit a high-quality reflective surface. However, when these films are examined under polarized light, while each grain of the film appears reflective at some angle of the polarizer, confirming that the chain direction of each grain lies in the plane of the film, the optical anisotropy of the film is substantially reduced when compared with single-crystal results. Moreover, these films show no evidence of electrical conductivity anisotropy at any temperature over the range of from close to 0.degree. K. to about room temperature.